A similar thing happened to me and somewhat concerned me when I first started dating my current boyfriend. I never said anything to him about it but when we become official/exclusive (right at the 3 month mark) he canceled his dating website membership on his own.
I'm so glad that I never mentioned it to him because the fact that
he made the decision to delete it (when he didn't even know I knew about it!) speaks volumes. And yes I did check to see if it was deleted about a week after he asked me to be his girlfriend, which is how I know, lol.
Anyway, if I were you I definitely wouldn't even bring it up unless you become exclusive, and then wait and see if he deletes it on his own... Then if he is still checking them frequently and a month or so goes by without him deleting them, I would probably be concerned and mention that your sister saw him online.
Good luck and I hope it works out!